Stabilized electroless plating solutions

ABSTRACT

An electroless nickel or cobalt plating solution is characterized by the addition of a small but effective amount of a source of mercury ions for increased bath stability. It is known in the art that solutions for electroless plating are unstable and tend to decompose with use. It is also known that decomposition can be retarded and the useful life of a plating solution increased by the addition of various additives, frequently catalytic poisons, in very small concentrations. In accordance with the present invention, it has been found that the stability of an electroless nickel or cobalt plating solution can be substantially increased by the addition of a source of mercury ions alone as a primary stabilizer, or preferably, by the addition of the mercury ions as a secondary stabilizer in combination with a prior art stabilizer as primary stabilizer. The combination of stabilizers provides a synergism with stability substantially improved over that obtainable with either component of the combination alone. One characteristic of the present invention is the co-deposition of mercury in minute amounts with the plating metal.

United States Patent 1 Gulla et al.

[541 STABILIZED ELECTROLESS PLATING SOLUTIONS [75] Inventors: Michael Gulla, Newton; Oleh Borys Dutkewych, Medfield, both of Mass.

[73] Assignee: Shipley Company Inc.,

Mass.

[22] Filed: Nov. 16, 1971 [21] Appl. No.: 199,307

Newton,

Related U.S. Application Data [60] Continuation'in-part of Ser. No. 65,301, Aug. 19, 1970, which is a division of Ser. No. 785,350, Dec. 19, 1968, abandoned.

Schneble et a1. ..106/1 [451 Feb. 20, 1973 Primary Examiner-Lorenzo B. Hayes Attorney-Robert L. Goldberg [5 7] ABSTRACT An electroless nickel or cobalt plating solution is characterized by the addition of a small but effective amount of a source of mercury ions for increased bath stability. It is known in the art that solutions for electroless plating are unstable and tend to decompose with use. It is also known that decomposition can be retarded and the useful life of a plating solution increased by the addition of various additives, frequently catalytic poisons, in very small concentrations. In accordance with the present invention, it has been found that the stability of an electroless nickel or cobalt plating solution can be substantially increased by the addition of a source of mercury ions alone as a primary stabilizer, or preferably, by the addition of the mercury ions as a secondary stabilizer in combination with a prior art stabilizer as primary stabilizer. The combination of stabilizers provides a synergism with stability substantially improved over that obtainable with either component of the combination alone. One characteristic of the present invention is the codeposition of mercury in minute amounts with the plating metal.

17 Claims, No Drawings STABILIZED ELECTROLESS PLATING SOLUTIONS CROSS REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a nickel or cobalt depositing composition and more particularly, to an electroless nickel or cobalt plating solution having extended life and temperature tolerance and characterized by the addition of a stabilizer comprising a source of mercury ions alone or in combination with a prior art stabilizer.

2. Description of the Prior Art Electroless metal deposition refers to the chemical plating of a metal over an active surface by chemical reduction in the absence of an external electric current. Processes and compositions useful therefor are known, are in substantial commercial use, and are described in numerous publications. For example, compositions for depositing electroless nickel are described in U.S. Pat. Nos. 2,690,401; 2,690,402; 2,762,723; 2,935,425; 2,929,742; and 3,338,726. Other metals which may be deposited electrolessly include cobalt, cobalt-nickel and cobalt-nickel-iron alloys, and palladium. Formulations for the deposition of some of the above electroless metals and others are disclosed in the 35th Annual Edition of the Metal Finishing Guidebook for 1967, Metals and Plastics Publications, Inc., Westwood, N.J., pages 483 to 486. All of the foregoing publications and patents are included herein by reference.

Known electroless nickel and cobalt deposition solutions generally comprise at least four ingredients dissolved in a solvent, usually water. They are (1) a source of the metal ions, (2) a reducing agent such as hypophosphite for nickel and cobalt (3) an acid or hydroxide pH adjuster to provide required pH, and (4) a complexing agent for metal ions sufficient to prevent their precipitation in solution. A large number of suitable complexing ions for electroless metal solutions are described in the above noted publications and also in U.S. Pat. Nos. 2,874,072; 3,075,586; and 3,075,855 also incorporated herein by reference. In some formulations, the complexing agent is helpful but not a necessity.

Although electroless nickel and cobalt solutions have been used for many years, the commercially used formations have not been fully satisfactory for several reasons. Among these are relatively slow deposition rates and bath instability. It has been shown that plating rate is dependent to some extent, upon the concentration of the reducing agent in the plating solution and that increased concentration will generally result in an increased rate of deposition. However, increased concentration of reducing agent also results in decreased bath stability. This is evidenced by a decrease in the time in which the plating solution will undergo uncontrollable decomposition (trigger).

It is known in the art that certain additives or inhibitors added to an electroless solution in properly controlled trace quantities act as stabilizers and retard the rate of bath decomposition. Generally speaking, these additives, or stabilizers are they are referred to in the art, are catalytic poisons. The concentration of the stabilizer in solution is usually critical. Trace quantities, typically in the range of a few parts per million, provide stability. An excess of stabilizer will partially or totally stop deposition of the electroless metal.

STATEMENT OF THE INVENTION The present invention is predicated upon the discovery that the addition of a small but efiective amount of a source of mercury ions to substantially any electroless nickel or cobalt solution improves stability without retarding the rate of deposition and in some solutions, provides a somewhat improved rate of deposition. Moreover, in accordance with the invention, it has been found that the addition of a combination of a mercury compound with a prior art stabilizer to an electroless solution provides a synergism resulting in a substantially increased solution stability. Accordingly, the present invention provides an electroless nickel or cobalt deposition solution comprising (1) a source of metal ions, (2) a reducing agent therefor such as hypophosphite, (3) a pH adjuster, (4) a complexing agent for the metal ions sufficient to prevent their precipitation in solution where necessary, and (5) a stabilizer for the solution which may be a source of mercury ions alone, as a primary stabilizer or in combination with a prior art stabilizer. Nickel and cobalt deposits from the electroless solutions of this invention are characterized by the co-deposition of small quantities of mercury.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As noted above, the mercury ion is believed to .be responsible for increased bath stability. Both simple and complex mercury compounds soluble in the electroless plating solution are suitable sources of the mercury ion for purposes of the present invention. Typical examples of mercury compounds include mercurous acetate, mercuric acetate, mercuric benzoate, mercuric chloride, mercuric bromide, mercuric carbonate, mercuric chlorate, mercuric chloride, mercuric iodate, mercurous nitrate, mercuric nitrate, mercuric sulphate, mercurous ammonium chloride, and mercuric ammonium chloride.

It should be noted that many mercury compounds are not fully soluble in aqueous solutions and many are considered insoluble. However, the mercury compound, for purposes of the present invention is required in amounts capable of dissociating to yield mercury ions in concentrations of only parts per million. Consequently, mercury compounds considered insoluble in aqueous solution may be soluble to the extent that they yield mercury ions in concentrations sufficient for purposes of the present invention. Mercury compounds soluble in aqueous solution are preferred.

Since the mercury ions are not catalytic poisons, their concentration in solution is not critical. Frequently, trace quantities are suitable. A preferred range comprises from about 1 to parts per million. However, amounts up to saturation in solution may be used or amounts in excess of saturation may be used, the excess serving as a source of mercury ions for replenishment.

The particular mercury compound selected for purposes of the present invention is not critical provided it yields sufiicient mercury ions and provided that the anion of the compound is not harmful to the plating solution.

In a preferred embodiment of this invention, the mercury compound is used as a stabilizer in combination with a prior art stabilizer. Materials known to the art as catalytic poisons to the deposition of electroless metal are frequently used in controlled amounts as stabilizers for electroless plating solutions. Perhaps the most widely used group of compounds of this nature are the divalent sulphur-containing compounds, many of which are disclosed in US. Pat. No. 3,361,540, incorporated herein by reference. Representative examples of such sulphur compounds are the inorganic sulfides such as sodium sulfide, potassium sulfide, sodium polysultide, and potassium polysulfide; organic and inorganic thio compounds such as sodium thiocyanate, potassium thiocyanate, potassium dithionate, sodium thiosulfate, and potassium thiosulfate; and organic sulphur containing compounds such as thiourea, 2- mercaptobenzothizaole, l ,Z-ethanedithiol, 1 ,2- benziosothiazane, methionine, 2,2'-thiodiethanol, dithioglycol, and thioglycollic acid.

The amount of the sulfur compound used in combination with the mercury compound is small and will vary depending upon the particular compound used. Typically, the sulfur compound is present in an amount less than that which will not stop deposition of the platin g metal. Generally, the amount may vary from a trace to about 300 parts per million dependent upon the sulfur compound used.

There are many stabilizers in the prior art in addition to divalent sulfur compounds. One other class of stabilizers comprises the water soluble cyanide compounds defined broadly to include nitriles and dinitriles set forth in US. Pat. No. 3,310,430. Typical of such compounds are alkali metal cyanides such as sodium and potassium cyanide; nitriles such as alpha-hydroxynitrile, e.g., glyconitrile and lactonitrile, and dinitriles such as iminodiacetonitrile and 3,3- iminodipropionitrile. The cyanide compound is used in an amount about equal to that of the divalent sulfur compound.

A class of stabilizers heretofore used for electroless copper solutions is disclosed in US. Pat. No. 3,457,089. These stabilizers are acetylinic compounds corresponding to one of the following generic formulas:

where each R is individually selected from the class of lower monovalent hydroxyalkyl, cyclohydroxyalkyl or hydroxyalkyl ether. Examples include ethynyl cyclohexanol, methyl butynol, methyl pentynol, dimethyl hexynol, 2-butyne-l,4-diol, dimethyl hexynediol, propargyl alcohol, hexynol and ethyl octynol.

It has been found that the above class of compounds can be broadened to include substantially more members when used in combination with a mercury compound and can be used in electroless nickel and cobalt solutions in addition to copper. For example, when used in combination with a mercury compound, each of R,R and R" can be aryl or aliphatic including cycloaliphatic substituted with a water solubilizing group such as hydroxyl and carboxyl. For purposes of the present invention, the acetylinic compounds contemplated will be referred to by the term solution soluble acetylinic compounds.

iodate compounds such as sodium and potassium iodate are useful stabilizers for electroless nickel solutions. It is believed that the use of an iodate compound as a stabilizer is not known to the prior art and that electroless nickel solutions containing an iodate compound are themselves novel compositions of matter. The Pb ion is also a useful stabilizer for electroless nickel solutions. Both iodate and Pb,.., ions are used in amounts of a few parts per million, preferably less than 50 parts per million.

An electroless nickel or cobalt plating solution stabilized with a mercury compound in accordance with this invention is used to deposit metal in the same manner as any prior art electroless solution. The surface of the part to be plated should be free of grease and contaminating material. Where a non-metallic surface is to be plated, the surface area to receive the deposit must first be sensitized to render it catalytic to the reception of the electroless metal as by the well-known treatment with an acidic aqueous solution of stannous chloride followed by treatment with dilute aqueous acidic solution of palladium chloride. Alternatively, extremely good sensitization of non-metallic surfaces is achieved by contact with a colloid of a precious metal having a protective stannic acid colloid and formed by the admixture of stannous chloride and a precious metal chloride, preferably palladium chloride, the stannous chloride being present in stoichiometric excess based upon the amount of precious metal chloride.

The invention will be better understood by reference to the following examples. where stability of solution was measured by the time it takes for a bath to spontaneously decompose (trigger) when plating a catalyzed cloth at one-half square foot per gallon. In all examples, catalyzed cloth was prepared by treating a cotton fabric according to the following sequence of steps:

1. Rinse cloth in a 20 percent (by weight) ammonium hydroxide solution maintained at room temperature for 5 minutes. Rinse in cold water.

2. Rinse for 5 minutes in 20 percent acetic acid solution maintained at room temperature. Rinse in cold water.

3. Immerse for from 20 to 40 seconds in a sensitizing solution of a palladium colloid having a protective stannic acid colloid (Catalyst 6F) maintained at room temperature. Rinse in cold water.

4. lmmerse for l to 3 minutes in a dilute hydrochloric acid solution maintained at room temperature. Rinse in cold water.

5. Dry cloth and cut to size.

In the following examples where rate of metal deposition from solution is recorded, rate was determined by plating over a phenolic substrate using the following procedure:

1. Cut phenolic board to a size measuring 2 inches by 2 inches.

2. Scrub clean with an abrasive cleaner. Rinse in cold water.

3. Treat for from 1 to 3 minutes with a non-ionic surfactant conditioner maintained at room temperature. Rinse in cold water.

minutes.

7. Rinse, dry parts and measure thickness of deposit. In the examples, Example number and Bath number are used interchangeably.

hydrate EXAMPLES l 16 Nickel sulfate hexahydrate Sodium hypophosphite mono- Sodium acetate Water Temperature 15 gm 20 gm to 1 liter (approx.)

Catalyzed cloth was plated by immersion in the above formulation containing stabilizers in amounts and with results as set forth in the following table:

STABILIZING AGENTS (PPM) Bath Time to Trigger No. Mercury Other (min) I l 2 2 mercuric acetate 7 9 3 Z-mercaptobenzo- 3 4 thiazole (2) 4 mercuric acetate 2-mercaptobenzo- 20 (20) thiazole (2) 5 methylbutoxyethanol 2 3 6 mercuric acetate methylbutoxyethanol I0 7 lead acetate (2) 2 3 8 mercuric acetate lead acetate (2) 9 thiodiethanol 6 10 mercuric acetate thiodiethanol l9 21 (20) (100) l l butynediol (20) 2 3 12 mercuric acetate butynediol (20) I0 12 13 acetylenedicar- 2 2 boxylic acid (50) 14 mercuric acetate acetylenedicarboxl0 I2 (20) ylic acid (50) 15 potassium acid 12 l5 iodate l5 l6 mercuric acetate potassium acid iodate (IS) 60 EXAMPLES 17 20 Nickel chloride dihydrate gm Sodium hypophosphite monohydrate 15 gm Sodium acetate 3 gm tol liter Temperature 175 200F pH 5 (approx.)

Bath

Mercury Other STABILIZING AGENTS (S) (PPM) Remarks 17 Heavy sidewall deposits after l hr.

18 mercurous Slight sidewall deposition after 1 hr.

acetate 20) I9 methylbutoxy- Slight sidewall deposition after 1 hr.

ethanol (25) mercurous methylbutoxy- No sign of sidewall deposition after 1 hr. acetate 20) ethanol (25) EXAMPLE 21 Nickel chloride dihydrate 30 gm Sodium hypophosphite monohydrate 10 gm Ammonium chloride gm Ammonium hydroxide to pH 8-9 Water to 1 liter Temperature 200 F The above formulation used to plate catalyzed cloth exhibited heavy sidewall deposition after 1 hour. With the addition of 20 parts per million mercuric acetate to a fresh solution of the same formulation, no sign of solution decomposition was evident after use to plate catalyzed cloth after a period of 1 hour.

EXAMPLE 22 Cobalt sulfate hexahydrate 30 gm Nickel sulfate pentahydrate 3 gm Sodium citrate 35 gm Ammonium chloride 50 gm Sodium hypophosphite monohydrate 20 gm Water to 1 liter Temperature 175 200 F pH 9 l0 Plating of catalyzed cloth with the above formulation resulted in sidewall deposition within a period of 1 hour. With the addition of 20 parts per million mercurous chloride to a fresh solution of the same formulation, no evidence of solution decomposition was witnessed after 1 hour.

EXAMPLES 23 24 The nickel and nickel-cobalt deposits of Examples 21 and 22 respectively, with and without the mercury addition were analyzed for the presence of mercury. In the formulations free of a mercury additive, mercury was not detectable in the deposit In the deposit from the formulation of Example 21 containing mercuric acetate, 0.19 percent mercury was found by analysis. In the deposit from the formulation of Example 22 containing mercury acetate, 0.150 percent mercury was found in the alloy deposit.

From the above, it should be readily apparent that the following advantages are gained from the addition of a source of mercury ions to an electroless metal solution.

1. Mercury ions used alone are primary stabilizers for electroless nickel and cobalt solutions.

2. Mercury ions in combination with prior art stabilizers for electroless solutions exhibit a synergism that improves stability of solution to an extent greater than that obtainable with either the mercury ions or the prior art stabilizer used alone.

3. The addition of a mercury ion to an electroless nickel or cobalt plating solution does not significantly affect the rate of deposition, as with other stabilizing agents known as catalytic poisons.

to stabilize electroless metal plating solution provided it is not associated with an anion detrimental to electroless plating of the metal (such as the sulphur or cyanide anion in larger than tolerable amounts) and provided further that the mercury source is soluble to the extent that it yields at least a few parts'per million of mercury ions in solution.

7. Mercury co-deposits with the electroless metal being plated. I

The reason for improved bath stability resulting from the addition of a source of mercury ions to the electroless metal solution is not fully understood, but is believed to be associated with the evolution of hydrogen gas as the metal deposits. An unusual property of mercury is its high overvoltage for hydrogen and it is believed that this property is associated with the stabilization mechanism.

It should of course be understood that changes may be made in the specific embodiments described herein without departing from the scope of the invention as defined by the following claims.

We claim:

1. In an aqueous electroless metal plating solution of nickel, cobalt or mixtures thereof including a source of the metal ions and hypophosphite as a reducing agent therefor and a complexing agent, the improvement comprising mercury ions in solution in a small but effective amount to provide increased bath stability.

2. The composition of claim 1 where the mercury I ions are derived from a mercury salt.

3. The composition of claim 1 where the mercury ions are in solution in an amount varying from a trace to saturation.

4. The composition of claim 1 where the mercury ions are in solution in an amount varying from 1 to 100 parts per million parts of solution.

5. The composition of claim 1 in combination with a second stabilizing agent for the electroless metal solution. a

6. The composition of claim 5 where the second stabilizing agent is selected from the group consisting of a divalent sulphur compound, a cyanide compound including nitriles and dinitriles, lead ions, iodate compounds and acetylinic compounds.

7. The composition of claim 6 where the acetylinic compounds corresponds to one of the following general formulas:

where each of R,R', and R" are radicals selected from the class of monovalent hydroxyalkyl, cyclohydroxyalkyl, and hydroxyalkyl ether.

8. The composition of claim 1 comprising a source of nickel ions, hypophosphite as a reducing agent therefore, a pH adjuster and mercury ions in solution in a small but effective amount capable of providing increased bath stability.

9. The composition of claim 8 in combination with a second stabilizer for electroless nickel solution.

10. The composition of claim 9 where the second stabilizer is selected from the group consisting of divalent sulphur compounds, lead ion, iodate compounds, and acetylinic compounds.

1 1. The composition of claim 10 where the acetylinic compound corresponds to one of the following formulas:

R-C I CH or RC E CR" where R, R and R" are radicals selected from the group of monovalent hydroxyalkyl, cyclohydroxyalkyl, and hydroxyalkyl ether.

12. The composition of claim 10 where the second stabilizer is lead ions.

13. The composition of claim 10 where the second stabilizer is a divalent sulfur compound.

14. The composition of claim 10 where the second stabilizer is an iodate compound.

15. A method for increasing the stability of an electroless nickel, cobalt" or nickel-cobalt alloy-hypophosphite plating solution and retarding the time in which the electroless plating solution spontaneously decomposes comprising the step of adding a mercury compound to said electroless plating solution in a concentration capable of providing a small but effective amount of mercury ions in solution.

16. An article of manufacture comprising a substrate coated with electroless metal-phosphorus alloy deposited from the solution of claim 1 and characterized by the co-deposition of mercury.

17. The article of manufacture of claim 16 where the electroless metal is predominantly nickel-phosphorus alloy. 

1. In an aqueous electroless metal plating solution of nickel, cobalt or mixtures thereof including a source of the metal ions and hypophosphite as a reducing agent therefor and a complexing agent, the improvement comprising mercury ions in solution in a small but effective amount to provide increased bath stability.
 2. The composition of claim 1 where the mercury ions are derived from a mercury salt.
 3. The composition of claim 1 where the mercury ions are in solution in an amount varying from a trace to saturation.
 4. The composition of claim 1 where the mercury ions are in solution in an amount varying from 1 to 100 parts per million parts of solution.
 5. The composition of claim 1 in combination with a second stabilizing agent for the electroless metal solution.
 6. The composition of claim 5 where the second stabilizing agent is selected from the group consisting of a divalent sulphur compound, a cyanide compound including nitriles and dinitriles, lead ions, iodate compounds and acetylinic compounds.
 7. The composition of claim 6 where the acetylinic compounds corresponds to one of the following general formulas: R - C * CH or R'' - C * C - R'''' where each of R,R'', and R'''' are radicals selected from the class of monovalent hydroxyalkyl, cyclohydroxyalkyl, and hydroxyalkyl ether.
 8. The composItion of claim 1 comprising a source of nickel ions, hypophosphite as a reducing agent therefore, a pH adjuster and mercury ions in solution in a small but effective amount capable of providing increased bath stability.
 9. The composition of claim 8 in combination with a second stabilizer for electroless nickel solution.
 10. The composition of claim 9 where the second stabilizer is selected from the group consisting of divalent sulphur compounds, lead ion, iodate compounds, and acetylinic compounds.
 11. The composition of claim 10 where the acetylinic compound corresponds to one of the following formulas: R - C * CH or R'' - C * C - R'''' where R, R'' and R'''' are radicals selected from the group of monovalent hydroxyalkyl, cyclohydroxyalkyl, and hydroxyalkyl ether.
 12. The composition of claim 10 where the second stabilizer is lead ions.
 13. The composition of claim 10 where the second stabilizer is a divalent sulfur compound.
 14. The composition of claim 10 where the second stabilizer is an iodate compound.
 15. A method for increasing the stability of an electroless nickel, cobalt or nickel-cobalt alloy-hypophosphite plating solution and retarding the time in which the electroless plating solution spontaneously decomposes comprising the step of adding a mercury compound to said electroless plating solution in a concentration capable of providing a small but effective amount of mercury ions in solution.
 16. An article of manufacture comprising a substrate coated with electroless metal-phosphorus alloy deposited from the solution of claim 1 and characterized by the co-deposition of mercury. 